Hey James, I am using a 3mm Diameter Straight Twin Fluted Wood Cutter. My current spindle speed is 12000 and feed rate is 12mm/sec. To be honest i really have no idea if i have the right settings, it was a half-educated stab in the dark as i have had no CNC training. When using thicker MDF the shapes seem to be coming out spot on but with the 4mm i am getting bad furring on the edges.